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Quick Hardening 

Concrete - 

A Practical consideration 

of products, methods, 
results and 


North American Cement Corporation 









Introduction ......"......... 3 

Fundamental requirements for good concrete work : 


Mixing 4 

Protection 5 

Acceleration: Possible — Practical — Economical 5 

Effect of accelerators on the strength of concrete 9 

Effect of curing on the strength of concrete 11 

Effect of water ratio on the strength of concrete 12 

Reducing the water through the use of Cal . 14 

Aids in frost-proofing and in densifying 15 

Use of Cal to correct unsoundness in concrete 16 

Comparative costs of concrete accelerators 18 

Calcium chloride in disguised forms 20 




THE constant development of new products and new 
methods in concreting, the constantly broadening use 
of Portland cement in every type of construction, the 
increasing variety of special conditions which it is called 
upon to meet, and the growing pressure for speed, have led 
us to prepare this brief consideration of the most economical 
and effective methods of handling concrete work in which 
early set and early strength are of importance. 

In recommending the use of our particular product, Cal, 
to meet certain requirements of modern concreting, we have 
carefully limited ourselves to those uses which the tests of 
experts and the experience of thoroughly capable and prac- 
tical men have amply demonstrated. 

The properties of Cal which are here generally dis- 
cussed may readily be applied by the reader to his individual 
problems of road and street building, reinforced concrete 
construction, stucco, brick mortar,* concrete products. 

Our technical staff will be very glad to consult with any- 
one interested in any phase of the concreting problems dis- 
cussed in this booklet. 

*The effect of Cal on mortar for use in brick const)~uction, is in 
general, the same as outlined in the following pages for concrete. It 
is important to note, in this connection, however, that five years of 
job experience have confirmed the finding of the U. S. Bureau of 
Standards (see Technological Paper No. 174, page 12) that the ad- 
dition of Cal to brick mortar does not cause efflorescence. 


Fundamental Requirements for Good Concrete Work 

N the making of the best Concrete and Mortar, there is 
no substitute for the selection of proper materials and 
insistence upon good workmanship.* 


Materials : 

1. Clean sand and stone. 

2. Well graded aggregates. 

3. Mixing water that is fit to drink 


1. The least amount of gauging water should be used 
that will give the workability necessary for the job. Keep 
reducing the amount of water until you find the point where 
the mixture is too stiff to work, and then add a little bit 
extra, (a pint often makes the difference in a 1-bag batch) 
until it can be handled easily. 

2. Long mixing. Mix concrete for at least one minute 
and mortar for at least two minutes. 

*Valuable facts have been gathered by the Portland Cement Asso- 
ciation on these points, and are fully given in its pamphlet entitled 
"Concrete Data for Engineers and Architects." Copies can be ob- 
tained free by application to any one of the numerous district offices 
of the Association, or to the headquarters at Chicago, III 


Protection : 

1. Protection from freezing. (See pp. 15-16). 

2. Protection from hot sun or drying winds. Just as 
soon as the concrete or mortar is placed, protect the exposed 
surfaces from the warm sun and from wind and other air 
currents, wherever job conditions make this possible, and 
continue the protection for several davs. 

Acceleration : Possible — Practical — Economical 

THE modern need for accelerating the hardening of 
Portland Cement mixtures is well presented in the 
following statement from Technologic Paper No. 174 
of the U. S. Bureau of Standards :* 

"A demand for a practical and efficient material for ac- 
celerating the hardening of Portland cement mixtures has 
received considerable attention in recent years. The need 
for such a material becomes of more consequence with the 
introduction of more rapid methods in construction and with 
the increasing amount of concrete repair and replacement 
work. Serious problems arise when it becomes necessary to 
interrupt or divert traffic for the duration of time required 
for concrete to harden sufficiently. The increased cost of 
lumber for forms has necessitated a more judicious use of 
such material, and its removal for further use as soon as 
the strength of the concrete will allow. This applies even 
more to the use of steel forms, which are rapidly replacing 
wooden forms in a great many types of concrete construc- 


~i 5 h 

"Considerable work toward the development of an ac- 
celerator for concrete has been done by the U. S. Bureau of 
Standards, which found that calcium chloride was the most 
effective of the various substances tested." 

The rate of acceleration to be obtained with calcium 
chloride is well know, and is generally accepted as adequate 
in all jobs except those which absolutely require a highly 
exceptional rate. However, as the paper above quoted goes 
on to say : " . . . . the use of commercial calcium chloride 
in concrete is attended by difficulties caused by its highly 
hj^groscopic property and by the handling of the solution." 

One of these difficulties is that calcium chloride is added 
in proportion to the volume of gauging water, which may 
vary from 25 to 45 gallons per cubic yard of a 1:2:4 mix, 
according to the dryness or wetness of the aggregates used, 
and without any relation to the amount of cement which 
the calcium chloride is intended to harden. The proportion 
is, therefore, extremely variable and the results necessarily 
uncertain. Too low a percentage of calcium chloride reduces 
the rate of hardening — too high a percentage may give 
'"flash set" and seriously affect the ultimate strength of the 

The second difficulty in using free calcium chloride is 

that, when it has been added to a barrel of gauging water. 

it disappears and leaves no trace which the man at the mixer 

an recognize. This invites the danger of a double dose, or 

no dose at all. 


The third difficulty is that free calcium chloride is highly- 
electrolytic and is, therefore, not advisable for use between 
or near electric railway tracks — where it may corrode the 
rails and also develop checks and cracks in the concrete 
from the electrolytic action. 

Chiefly as a means of making acceleration a safe, prac- 
tical and economical operation under the conditions of aver- 
age construction work, and with the grade of labor ordin- 
arily employed, our chemical staff developed a compound 
which uses calcium chloride as a base, yet eliminates to a 
large degree the amount of labor and the chances of error 
involved in mixing. 

This compound, Cal, is a dry white powder which re- 
quires no solutions or extra preparations, but is added di- 
rectly to the cement and aggregates, at the mixer. 

In the form of Cal, the corrosive properties of calcium 
chloride are entirely neutralized. Cal alone is actually 
an inhibitor of corrosion. 

Regarding the accelerative value of Cal, we quote again 
from U. S. Bureau of Standards Technological Paper No. 
174 (page 7) : 

"The effect of Cal on the setting time of normal Port- 
land cements may be seen by examing the results given 
below. The set was hastened in all cases and to a greater 
extent by the larger percentage of Cal." 




Ratio (percentage) 
Cal to cement 














4. . 




5. . 




Ratio (per 
water to 






H. M. 


2 15 
1 50 

3 35 
1 50 

1 25 








































From Technologic Paper 171. V. S. Bureau o1 Standards. 

The official conclusion drawn from the tests here tabul- 
ated, are given on page 22 of the above paper. 

"The setting of normal Portland cement mixtures may be 
hastened by Cal to an extent which is very desirable in 
concrete construction requiring a finished surface. The 
finishing operation may proceed with much less delay after 
the concrete has been placed, which should result in cut- 
ting down overtime labor. This hastening of the set is 

.• /■ 

I J 


- \ 


N - 






i 8 y 

not objectionable in any type of construction providing the 
concrete is placed soon after it is gauged with water. 

"Cal was used in a concrete floor topping alongside of 
topping containing no accelerator, for the purpose of ob- 
serving the effect under actual field conditions. The treated 
topping was ready for the finishing operation in about one- 
half the time required by the untreated, as judged by the 
finisher. This agreed very closely with the results of the 
laboratory tests on the rate of setting of the two mixes by 
means of the flow table." 

If the calcium chloride could be eliminated from Cal 
without reducing the effect on fatness and workability, and 
if full advantage were taken of the drier mix which this 
workability allows, a very considerable degree of accelera- 
tion would still be achieved. 

In the case of Cal, there is actually a double accelera- 

1. That produced by its calcium chloride content. 

2. That produced by the lower water ratio which its 
fattening effect makes possible. 

Effect of Accelerators on the Strength of Concrete 

WHETHER calcium chloride alone, or calcium chloride 
in the form of the Cal compound, be used, it has 
been established that a rapid increase of strength 
may be looked for. 

The conclusions with regard to the operation of Cal 

are summarized in the following quotation from pages 22 

and 24 of the U. S. Bureau of Standards Technologic Paper 
No. 174. 

~i 9 h 

"All Portland cement mixtures treated with Cal at- 
tained greater strength at the 2-day and 7-day periods than 
the corresponding untreated mixtures. The percentage in- 
crease in the strength of mortar at the 2-day period ob- 
tained by an addition of 5 per cent Cal to cement ranged 
from 40 to 140. The strength of the treated mortar at two 
days was equal to the strength of the untreated mortar at 
three and one-half to eight days. These calculations are 
made from the results of tests in which the test pieces were 
stored in water, damp sand, or damp closet. 

"Treated mortars stored in the laboratory air attained 
at 2 days strength greater than that of the untreated mor- 
tars at 28 days. This was due to the rapid drying out of 
the small test pieces and the comparatively slow rate of 
gain in strength after the 2-day period. However, this 
indicates that Cal is especially advantageous in cement 
mixtures which are necessarily subjected to any drying-out 

"The increase in strength produced by 5 per cent Cal 
in concrete mixtures at the 2-day period ranged from 52 
to 135 per cent, and the strength of the treated concrete 
at the 2-day period was equal to that of the untreated at 
from three to four and one-half days. On an average this 
represents a saving of approximately one-half the time in 
operations which are dependent upon the strength of the 
concrete at early periods. The effect of the air storage in 
the concrete tests was lessened in degree, owing to the high 
relative humidity which existed throughout the storage 

"It should be remembered that the increase in strength 
resulting from an addition of 5 per cent Cal does not repre- 
sent the maximum which may be obtained except in very 


-4 i o y- 

rich mixes. As much as 15 per 
tar tests, giving an increase of 

cent Cal was used in mor- 
220 per cent at the 2-day 

Effect of Curing on the Strength of Concrete 

THE importance of controlling the conditions of curing 
concrete after it has been placed should not be over- 
looked. It has been well established that a moist at- 
mosphere at 70° F. presents the ideal curing condition for 
practically all types of concrete construction, and that the 
maintenance of this condition gives maximum strength, du- 
rability and density to the finished structure. Such ideal 
curing conditions, of course, can rarely be obtained on the 
actual job. Variations in temperature and in humidity may 
extend over wide ranges, and unless special precautions are 
used, the resulting concrete will be equally variable. 

The most severe condition which can be met in ordinary 
construction work during the curing period is produced by 
very hot and very dry weather. In such weather concrete 
is dried out rapidly from the surface, so that there is not 
enough moisture left to properly combine with the cement. 
The result is a soft-cured, porous and cracked surface, 
which offers easy entrance to rain, frost and other destruc- 
tive agents of the atmosphere. 

Where Cal is used with concrete cured under such dry- 
ing-out conditions as are described above, the results are 
particularly interesting and encouraging. They are sum- 
marized as follows in the U. S. Bureau of Standards Tech- 
nological Paper No. 174 Page 10) : 

"The effect of Cal vnder the conditions of these tests is re- 
markable. The strength of the treated mortar at S days is con- 
siderably above that of the untreated mortar at 28 days." 


H[ 1 1 t- 

The action of Cal under these drying-out conditions 
seems to be two-fold : 


It accelerates the set and strengthening of the con- 
crete or mortar so that the period in which serious 
drying-out is possible is reduced to the minimum. 

2. Cal has the property of retaining the water of the 
mix and resisting evaporation. 

For all work in which surfaces are necessarily exposed 
to sun and wind, this property of Cal will be found of real 

Effect of Water Ratio on the Strength of Concrete 

THROUGH the long and careful investigations of the 
Portland Cement Association, it has been demon- 
strated that the strength and durability of a finished 
concrete job depends in a very considerable degree upon the 
percentage of water used in the original mix. The official 
report of this Association makes the following statements: 

"Excess mixing water weakens concrete. Sloppy mix- 
tures sacrifice strength. One pint more water than neces- 
sary in a one-bag batch decreases the strength and resist- 
ance to wear of concrete as much as if two or three pounds 
of cement were left out. Concrete hardens because of chem- 
ical reactions between Portland cement and water. The 
quantity of mixing water is just as important as the quan- 
tity of cement. 











-Proper £€>n*jftrncy for mas* ca/itrrrfe, 
concrete A/itft>*&Y0ar t f/T?*'/7tj M efc 

r h/s rat??* &fc&rts/ste/icy sAwfe/ 

Ar used for cast pnxf acts; r€/r?fbnceo 
cpncrcfe, stcjt&f/? members raqu/ra 

\/fte frt&fer amount &f prsfrr 

•With this consistency about" 
one-iha/f the strength h~ tost 

With the sfoppy "concrete some - 
'•nes used in rood work and /n 
but. 'ding construct/on, t#o- thirds 
to three-fourths of the possio/e 
strength of the concrete is hst 

ISO (60 ITO /SO (30 tOO 

fO iO *0 tOO HO fZO 130 14-0 t&v ivv rtv wv rx too 

Wtter Used- Figures are percent- of Cuantity Giving "Maximum ftrength^ 

Fig. l. Effect of Quantity o£ Mixing: Water on the Compressive Strength 
of Concrete. Note: In general construction, the maximum strength can 
rarely he obtained, but it is possible to obtain 70 to 90 per cent of the 
maximum strength without additional expense by restricting the quantity 

of mixing water. 

With given proportions, the 
should obviously be reduced as 
tain a plastic mixture. 

"The strength of 
concrete depends on 
the ratio between 
the volume of mix- 
ing water and the 
volume of cement 
( J ). As long as 
the mixture is work- 
able, the smaller the 
water-cement ratio, 
the stronger the 
concrete. Thus de- 
creasing the quan- 
tity of mixing water 
and increasing the 
quantity of cement 
both add to the 
strength of concrete, 
quantity of mixing water 
far as possible and still ob- 

"The curve in Fig. 1 shows the effect of the quantity of 
mixing water on the strength of concrete. It is an average 
of the results obtained with many mixtures and aggregates. 
The data on which it is based were secured from several 
thousand tests at the Structural Materials Research Labor- 
atory, Lewis Institute, Chicago, 111. Mixtures on the right 
of the maximum strength line were plastic; mixtures on 
the left were not. 

"In general construction work, maximum strength can 
seldom be secured, because the mixture would be too stiff 

to be workable, but 70 to 90 per cent of the maximum 
strength can readily be obtained. This would be a great 
increase over the usual results, since much of the concrete 
placed today contains 50 to 100 per cent more water than 
necessary and thus attains only half or even only a quar- 
ter of its possible strength. 

"Resistance to wear, which is vital in concrete pave- 
ments, floors and sidewalks, increases with compressive 
strength. Thus, the smaller the quantity of mixing water 
the more wear-resistant will be the concrete." 

Reducing the Water Ratio Through the Use of Cal 

As an aid to reducing the water content, Cal will be 
found efficient. One of its ingredients is a specially precipi- 
tated hydrated lime which acts as a lubricant to the mix, 
making it "fatter" and more easily workable — so that a mix 
with less than the usual amount of water will flow quite as 
readily and be as easily handled on the shovel or trowel, or 
through a chute. The additional fatness and workability 
given by Cal produce a mix that reaches the bottom of 
the chute in the same uniform condition, with the same 
ratio of cement and aggregates throughout that it had 
when it left the top. 

In this connection, it is important to note that Pearson 
and Hitchcock, after making an exhaustive test of worka- 
bility came to the conclusion that hydrated lime in the form 
in which it is present in Cal is superior to ordinary dry 
hydrated lime in effecting workability of the mix and thus 
reducing the water ratio required. 

■i u h 

Their conclusions are expressed as follows on Page 293 
of the Technical Papers (Part II, Vol. 23, 1923) of the 
American Society for Testing Materials: 

"Accelerator 'A'* is a calcium oxychloride which quickly de- 
composes to calcium chloride and hydrated lime when mixed 
with water. The hydrated lime thus freed is much more effec- 
tive upon workability, as indicated by the results of these tests, 
than the dry hydrated lime." 


Reference again to the U. S. Bureau of Standards table 
quoted on Page 8, shows that with an 8% addition of 
Cal the water content for usual flow is about 2*4% less 
than required for mixes to which no Cal has been added. 

Aids in Frostproofing and in Densifying 

FOR lowering the freezing point of the mix, and hasten- 
ing its set so that it is exposed for a shorter period to 
the possibility of freezing, Cal will be found a valu- 
able and dependable aid. 

In general, Cal will render mortar or concrete proof 
against freezing at 10 degrees below the temperature at 
which the untreated mix could safely be used. 

_ When concrete is placed at low temperatures, the most 
critical period of setting is during the first few hours im- 
mediately after the concrete is poured. After the setting 
action is well started, there is far less likelihood of damage. 
Cal is particularly effective at the earliest stages after the 

-4 15* 

concrete is poured in hastening- the setting- action of the 
cement. Also, because of its fatness and the consequent 
absence of "free water" in properly mixed Cal-Concrete, 
the possibility of freezing is still further reduced. 

The use of Cal should not, of course, cause neglect of 
the well-recognized precautions for concreting in extreme 
weather, such as heating aggregates, heating water, cover- 
ing finished work, etc. 

The water-proofing characteristics of Cal are chiefly 
the result of the specially precipitated hydrated lime which 
forms an important part of the compound, and which in 
a less effective form has long been used independently fo 
this purpose. The fatter mix obtained with Cal insures 
more compact and more complete filling of molds and forms, 
and closes up the pores of the concrete with verj r fine par- 
ticles of hydrated lime. Its chemical action insures a thor- 
ough cure and the complete hydration of the cement through 
and through which is necessary for perfect and lasting 

Use of Cal to Correct Unsoundness in Cement 

IN cases where the steam test shows an inferior quality 
of cement and where circumstances make it impossible, 
difficult or unduly costly to wait for a fresh shipment, 
the unsoundness can be corrected by the use of Cal. This 
has been amply demonstrated in actual field operations, and 
is authoritatively expressed as follows by the U. S. Bureau 
of Standards in Technologic Paper No. 174 (Page 9) : 

m -» 


"A very unsound cement was used in a 1:3 mortar which 
was molded into 2-inch cubes and stored in air after 24 
hours. At the end of 8 months, the plain mortar test pieces 
were entirely disintegrated. The test pieces made from the 
same mortar to which 5 per cent of undried Cal was added 
were still very firm, although a few disintegration cracks 
had developed." 

tory No. 

Soundness after addition of various percentages of undr ied Cal 

per cent 




4 3200 

5 ... 

_ per cent 

warped, not 

very soft 


Sound; very 

Unsound ; 

Unsound ; 
warped, not 


5 per cent 

Sound; very 

Sound; very 


8 per cent 


. . .do 

Sound; slight- 
ly warped, 

Sound; very 



ram Technologic Paper 17 A, U. S. Bureau of Standards. 

The conclusion from these tests is expressed on page 22 
of the above paper in these terms: "Unsound cements may 
be greatly benefitted or made sound by an addition of Cal." 


Comparative Costs of Concrete Accelerators 

IF one considers only the cost of the actual product re- 
quired to produce a given degree of acceleration, com- 
mercial calcium chloride is, of course, cheapest. This 
chemical may be obtained in the granular or flake form, and 
is made into water solution before use. Its cost, when care- 
fully used, is about 17 cents per barrel of cement treated, 
as compared with a cost of 60 cents for Cal, costs of $1.00 
to $4.00 for the various "patent medicine" forms of Calcium 
Chloride, and about $4.50 per barrel more than Portland 
Cement for special quick hardening cements of the high 
alumina type. 

For work where acceleration only is sought, and where 
the size and stationary character of the job permits the 
establishment of adequate equipment for the convenient 
storage and handling and the accurate proportioning and 
testing of the accelerator, calcium chloride will give satis- 
factory results at minimum cost. 

In a large number of cases, however, the use of Cal, at 
the increased cost, will be justified. This is especially true 
where the concrete mixer is in motion (as in road and street 
work), where water connections have to be changed re- 
peatedly, and where there can be no permanent provision 
for the preparation, storage, and analysis of the accelera- 
tor. On such jobs, the saving of time and the assurance of 
accurate proportioning alone warrant the extra cost. 


A 1 8 * 

In addition, however, it must be borne in mind that for 
the actual amount of calcium chloride contained in Cal, 
a higher degree of acceleration can be obtained because of 
the drier mix which Cal permits and the increased setting 
speed produced by this absence of the "free water" which 
retards hardening. 

And, finally, where the ultimate durability of the con- 
crete is given proper consideration, the increased fatness 
and improved cure of a mix obtained with Cal, will insure 
a permanence and quality which makes its increased cost a 
comparatively small item. 

For the use of the "patent medicine" compounds dis- 
cussed on the next page there is no real justification in re- 
i Us for the high costs fixed on these products. 

In those exceptional cases where very high strengths 
are needed in 24 hours, it will often be found advisable to 
use special cements, but it is always necessary to compare 
costs with the actual saving in time which can be realized. 
Frequently it will be found that Cal and Portland cement 
will give all the strength necessary for the particular job 
in very little more time than the special cements. 

Calcium Chloride in Disguised Forms 

IT is a matter of common knowledge that herbs and :hem- 
icals of proved medicinal value and comparatively low 
cost are largely sold in compounds bearing a trade name 
at a tremendous increase in price. 


The same thing- has happened in the case of calcium 
chloride. Its acknowledged property of acceleration, and the 
protection this gives against freezing, has caused it to be 
used as the principal ingredient of a great variety of trade- 
marked products sold under names suggestive of these de- 
sirable accelerating and frost-proofing results. 

Practically all these "patent medicine" products have one 
merit — the merit that results from the net amount of actual 
calcium chloride which they contain. But it will be found, 
upon chemical analysis, that this merit is the only one 
which they possess — and that other ingredients are added 
merely to disguise the base and to make plausible the tre- 
mendous increase in price over the cost of the calcium 

chloride. This increase runs anvwhere from 300% to 

While Cal is also a compound with a base of Calcium 
Chloride its exact constituents have been made a matter of 
public record through the analysis of the U. S. Bureau of 
Standards, given in Technologic Paper No. 174 and its added 
advantage of convenience, safety, labor-saving, double ac- 
celeration, and improved durability in the resulting concrete 
give ample warrant for its higher cost as compared with 
ordinary calcium chloride. 







Technologic Papers 

Bureau of Standards 

6. W. 8TRATTOW. Di»»cto» 

No. 174 




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